This invention relates generally to carbon and graphite electrode joints, and more specifically to the carbon and graphite nipples which are commonly used to assemble such joints.
Carbon or graphite electrodes of the type utilized in electric furnaces and similar metallurgical environments are consumed in use thereof, and accordingly electrodes are normally formed from abutting electrode sections which are interconnected by a nipple internally threaded into each of the adjoining sections. In order to assure electrical and mechanical integrity of the resultant joint, various methodology has been developed, the objective of which is to provide some type of cementing action at at least a portion of the thread clearance spaces whereat the nipple engages the abutting electrodes.
According to one of the most common and highly successful techniques used, a reservoir is formed at the connecting nipple during manufacture thereof and/or at one or more portions of the abutting electrode sections. A cementitious composition, most commonly pitch, is deposited in such voids prior to formation of an electrode joint. Upon the formed joint being thereafter subjected to heating, the composition as, for example, the cited pitch, flows from its reservoir into some of the adjacent thread clearance spaces, where it subsequently is carbonized by further heating as to provide a permanent bond having both good electrical and mechanical properties. Nipples of the reservoir-containing type thus referred to, are shown, for example in U.S. Pat. Nos. 2,510,230; 2,735,705; 2,941,828; 2,941,829 and 3,419,296. The various nipple configurations shown in these exemplary disclosures may include, for example, bored radial recesses extending inwardly from the nipple periphery, or other configurations of cavities in which the so-called locking pitch, may be deposited.
As has already been indicated, the most common composition for use with reservoir nipples has indeed been pitch itself, which commonly is deposited within the reservoirs by heating the pitch to a temperature rendering it fluid, and then allowing the molten material to flow into the reservoir where it subsequently hardens. This technique of preparing the nipple product is, however, undesirable from a number of viewpoints. Not only, for example, must relatively cumbersome and costly heating and handling equipment be thus used, but moreover large quantities of noxious and toxic fumes are generated in the process -- which constitute a danger both to the workmen and to the community at large. In addition the corrosive action of the molten pitch severely limits the useful life of the equipment. Furthermore, the resultant nipple product is less than satisfactory in that the distribution of the re-deposited pitch upon subsequent melting is not entirely predictable, depending primarily upon random flow provided during the melting process.
It may be noted that within recent years compositions other than pitch alone, have been proposed for use with the reservoir-type nipples cited above. For example, in U.S. Pat. No. 3,624,011, a cement is disclosed based upon a mixture of hardenable synthetic resin with pitch and dextrin. However, in this composition, as well as similar compositions, it is contemplated that the binder constitute a thermosetting material. This, in turn, impedes the flow and redepositing sequence which is sought to be obtained upon heating of the composition.
It has from time to time also been proposed that the distribution problem cited above may be alleviated by a use of agents in the composition which serve to expand the composition upon heating, thereby forcing such composition into the thread clearance spaces. For example, in U.S. Pat. No. 3,140,967, a technique is disclosed wherein a dissolvable plastic bag is positioned at alternate ends of the nipple in an electrode assembly. During heating the bag ruptures, releasing the cementitious material. The latter includes a swelling agent which presumably acts as set forth above. The composition utilized, however, similar to that mentioned in the prior paragraph, is thermosetting, and in fact it is contemplated that the surfaces to be cemented be preimpregnated with a catalyzer to promote hardening. The effect of this arrangement is one of counteracting the very augmentation of flow which is sought to be achieved by the swelling agent. Placing the materials at the indicated alternate ends of the nipple, moreover, is found in practice to be a relatively ineffective situs for enabling an effective distribution.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a locking pitch composition for depositing within the pitch reservoir of carbon or graphite nipples of the type including such voids, which composition is heat-foamable and thermoplastic, enabling excellent distribution of such material upon use of a thus-prepared nipple in a high temperature electrode assembly environment.
It is a further object of the present invention, to provide a locking pitch composition which may be deposited within the pitch reservoir of graphite nipples of the type including such voids, by direct and simple extrusion techniques, thereby eliminating the costly and cumbersome operations required to deposit such locking compositions in the past, and moreover eliminating the toxic and noxious fumes which are incident to the melting and pouring techniques of the prior art.
It is a further object of the present invention to provide a method for formation of a superior carbon electrode joint, wherein the pitch reservoir of the nipple used in such joint is filled by a simple extrusion technique which may be conducted at ambient temperatures with a material that will harden to be fully retained in the nipple reservoir prior to use and will flow and expand during use.
It is a further object to provide a locking pitch composition having a high coking value and which is substantially free of shrinkage when exposed to furnace temperatures.
It is a still further object of the invention, to provide a locking pitch composition, and method for use thereof which yields electrode joints displaying superior mechanical properties with respect to both strength and performance under flexure and thermal stress.